The present invention relates to optical time domain reflectometry and more specifically to high resolution examination of single mode optical fibers using multimode optical pulses.
Optical time domain reflectometers, OTDR, are used in the telecommunications industry for examining fiber optic cables to locate discontinuities or breaks that affect the signal transmission quality through the cable. In examining a fiber with an OTDR, optical pulses generated by a laser are launched into the fiber under test. During the time period between the optical pulse transmissions, light reflected back from the fiber in the form of backscatter and reflections associated with events, such as splices, coupler connections and breaks, is converted to an electrical signal, sampled and stored for display. The stored data is displayed as an amplitude vs time plot showing a gradually decreasing backscatter energy level with reflective events appearing as pulses on the backscatter.
There are two major types of optical telecommunication systems in use today, multimode systems and single mode systems. Each system has specific attributes based on the wavelength of the optical source and the core diameter of the optical fiber used. In a multimode system, short wavelength light in the range of 850 nm is generally used. The optical source for the light is either a light emitting diode, LED, or a laser diode. The most commonly used optical fiber has a core diameter of 62.5 microns with other sizes ranging from 50 to 100 microns. A major advantage in 850 nm based systems is the lower cost of components compared to single mode systems. However, signal attenuation is wavelength dependent so the transmission range of 850 nm multimode systems is limited. This is also true for a single mode system operating with a 850 nm optical source. Another factor affecting multimode systems is the dispersion effects on data transmission bandwidths over distance. As the transmission distance increases in a multimode system, the data bandwidth decreases.
Single mode transmission systems are used where greater transmission range and data transmission bandwidths are required. The transmission range in single mode systems is on the order of tens of kilometers and the data transmission bandwidth is currently in the range of several gigahertz. In a single mode transmission system, the core diameter of the fiber is approximately 9 microns. Longer wavelength optical sources that are less affected by fiber attenuation are used. The two most commonly used optical sources are laser diodes operating at 1310 nm or 1550 nm.
Due to wavelength dependent attenuation in the fiber and different core diameters used in the various optical telecommunication systems, electronic test and measurement equipment manufactures produce OTDR's specifically designed for testing each type of optical transmission system. An example of a multimode OTDR is the OF150, manufactured by Tektronix, Inc., Beaverton, Or., U.S.A., and the assignee of the present invention. The OF150 uses a 850 nm laser to generate optical pulses that are coupled through a multimode coupler to a front panel connector via multimode optical fibers. The front panel connector accepts 62.5 micron fiber for testing. Reflected light from the fiber under test is coupled to a silicon detector, which converts the optical signal to an electrical signal for further processing by the instrument. An example of a single mode OTDR is the OF235, manufactured by Tektronix, Inc. In the OF235, 1300 nm and 1550 nm lasers are provided for testing single mode transmission systems at either wavelength. Optical power is coupled through a single mode coupler to a front panel connector via single mode optical fiber. The front panel connector accepts a 9 micron fiber for testing. Reflected light from the fiber under test is coupled to a germanium detector, which converts the optical signal to an electrical signal for further processing by the instrument.
Multimode OTDR's are not capable of accurately testing single mode optical transmission systems. Significant signal loss occurs when trying to couple a single mode fiber to a multimode connector due to the core diameter difference of the respective fibers. This substantially reduces the dynamic range of the instrument. In addition, a significant signal reflection occurs at the interface between the fibers. Further, most event of interest are wavelength sensitive, so that losses at 850 nm are different from those at 1310 nm and 1550 nm. The signal loss is mode dependent, so some fiber features are attenuated more than others.
OTDR's designed for single mode applications have their own drawbacks. The germanium optical detectors used with longer wavelengths of light have a longer detector tail caused by detector storage effects than silicon detector used in OTDR's operating with 850 nm optical sources. The detector tail limits the ability of the OTDR to resolve events, such as reflections, that are close together. In addition, to obtain sufficient dynamic range in single mode applications at 1310 nm or 1550 nm wavelengths, relatively long pulse widths of light are used. This again reduces the resolution of single mode instruments using these wavelengths to detect events that are close together.
What is needed is an instrument for testing single mode optical transmission systems that provides high resolution for detecting events that are close together.